by: Martin Sagendorf
Imagine students’ amazement when they actually see sunlight melt a penny with the Atomic Penny Vaporizer! This demonstration clearly illustrates the vast amount of energy illuminating the Earth’s surface. In rough numbers: 70% of the Sun’s incident energy on our outer atmosphere is reflected back into space – only about 30% actually gets to the Earth’s surface. But, as we experience, this is still a substantial quantity of energy.
Fortunately, this energy (I. R. – Visible – U. V.) is rather uniformly distributed over the Earth’s surface – thus its overall intensity is such that we have a habitable environment. However, as we all know, we can concentrate some ‘area’ of this energy to increase the ‘energy per area’ (a measure of this is the temperature of the area of concentrated energy). A common magnifying lens (2-4 in. diameter) will concentrate sufficient energy to burn paper or other objects with a low flash point.
To achieve an even higher energy concentration it is only necessary to use a device that increases the ‘capture area’ of the Sun’s energy. Fortunately, this is easily accomplished with an inexpensive ($3.50) Fresnel lens. The Fresnel lens (64 sq. in.) used here will ‘collect’ nine times the energy of a 3 in. diameter magnifying lens –creating a ¼” diameter ‘spot’ of energy having a temperature of over 315 degrees C (600 degrees F). This is sufficient energy to melt zinc.
All that’s required are a Fresnel lens, bright sunlight, and a means of holding a penny. Actually, it is a bit
more involved: the Fresnel lens must be positioned exactly perpendicular to both the sunlight and the penny; the penny must be of 1982 or later; and the penny must be supported by a thermally minimally-conductive means.
Guidelines for building the Atomic Penny Vaporizer are detailed in the book Physics Demonstration Apparatus. This amazing book is available through Educational Innovations and includes ideas and construction details, including all equipment necessary, for the creation and use of a wide spectrum of awe inspiring physics demonstrations and laboratory equipment. Included are 48 detailed sections describing hands-on apparatus illustrating mechanical, electrical, acoustical, thermal, optical, gravitational, and magnetic topics. This book also includes sections on tips and hints, materials sources, and reproducible labels.
And, as a footnote, it is not illegal to ‘vaporize’ a penny or any other United States currency. By law, it is only illegal to alter U. S. currency if the intent is to defraud – melting a penny with the Sun’s energy is simply a wonderful example of energy and its effects – perfectly legal.
I’m thrilled to find your idea about using fresnel lenses to demonstrate the power of solar! I’ve been salvaging fresnels from overhead projectors and projection tvs for years but haven’t known exactly how to incorporate them into my science class. Do you happen to know what welding lens # would be sufficient to prevent eye damage?
Hi Greg, We asked Marty Sagendorf, the author of this article, and his advice follows.
The safest protection would be No. 12 Welder’s helmet lenses – they are small, however larger lenses are available (on the web). Again, the caution: NEVER look at the sun with the Fresnel lens or even through a welder’s lens. Hope this helps.
Tami
Educational Innovations